Resolve WordPress Upload Glitch

My site is throwing me a curveball: I need straight-up website troubleshooting. At the moment I can’t upload any custom HTML code and, as a result, some features and forms refuse to respond. I suspect a conflict buried somewhere in the theme, a plugin, or the core files, but I haven’t pinned it down... (Budget: $10 - $30 CAD, Jobs: cPanel, CSS, HTML, JavaScript, PHP, Web Development, Website Optimization, WordPress)